The invention relates to a method for processing the output signal of a low-noise sensor, particularly a sensor based on one or several Josephson junctions, such as a SQUID.
The invention also relates to an apparatus for processing the output signal of a sensor based on one or several Josephson junctions, such as a SQUID.
The invention also relates to a multichannel magnetometer.
The invention also relates to an apparatus for measuring the strength and/or gradient of a magnetic field.
The method and apparatus of the invention are particularly related to processing output signals from SQUID sensors and generally from other sensors with corresponding electric properties.
The SQUID sensor, or SQUID (Superconducting Quantum Interference Device) is used for measuring weak magnetic fields. The output impedance of a SQUID is only about 1-5 .OMEGA.. Its operative temperature is typically 4.2.degree. K., down to which temperature it is suitably cooled. The operative temperature depends on the superconducting material used in the SQUID, and is therefore as low as the superconductor needs in order to work. The noise in the output is very low; it is not much higher than the thermal noise of resistors used in attenuating Josephson junctions. The SQUID sensor is often used for measuring low-frequency signals within the range of 0.1 Hz-10 kHz.
A drawback in the sensors described above is that it is difficult to amplify the output signal without increasing noise. The reason for this is that the frequency fluctuation of the output signal is relatively small, about 10 uV-100 uV from peak to peak. Moreover, the noise of the SQUID in the output roughly corresponds to the thermal noise of resistors used in attenuating Josephson junctions. If the noise temperature of the amplifier following a SQUID with the temperature of 4.2.degree. K. is below 10.degree. K., the amplifier does not remarkably increase the incertainty in the measurement of the magnetic flux.
None of the ordinary dc-coupled amplifiers operated at room temperature have sufficiently low noise capacities when used directly with a SQUID sensor. This is due to the low output impedance of the SQUID, as well as to the fact that it is often used for measuring low-frequency signals within the area 0.1 Hz-10 kHz. Within the frequency range 1 kHz-100 kHz, the FET amplifiers have an extremely low noise temperature and therefore there is employed a flux modulator and a transformer functioning at low temperatures in connection with a dc SQUID sensor, in order to adjust the low impedance to the FET amplifier. When proceeding in this fashion, the uncertainty in the measurement of the magnetic flux is defined by the noise of the SQUID solely. Moreover, it can be proved both theoretically and empirically that when the noise from the SQUID is decreased, the magnetic flux-frequency modification grows, and consequently the requirements for the signal processing electronics are not increased.
However, problems arise when several SQUID sensors are coupled together to form multichannel devices. Among such devices, let us mention a multichannel magnetometer. They have been lately manufactured for measuring weak magnetic fields of the brains and the heart. At present the goal is to accomplish a magnetometer with 30-120 channels. If such a multichannel magnetometer is realized by means of SQUID sensors, by utilizing flux modulation techniques, the expenses caused by the signal processing electronics grow remarkably, because for each SQUID and channel there is needed a cryogenic transformer, a preamplifier, a modulator etc. The electronics in this kind of a multichannel magnetometer become really complicated and costly.
It is known that by using positive feedback, the amplification of the amplifier is grown. However, this method has the risk that the amplifier becomes unstable and/or is drifted aside from its operating point.
In addition to this, in connection with positive feedback it is difficult to realize the signal processing device so that the noise of the sensor, such as a SQUID sensor, were not dependent on the noise of the next amplification level, too. The reason for this is that to arrange sufficient amplification means to make the device nearly unstable. If, on the other hand, the device is designed to be extremely stable, the strengthened amplification caused by positive feedback is not enough to ensure that the noise of the next amplification level should not affect the total noise.